The present invention relates to portable communication systems, and in particular systems intended to provide remote or portable communication through a network such as the telephone line.
Even when remote or portable telephone access is provided, some degree of privacy is still desired, mitigating against the use of radio wave communication which can broadcast a conversation a great distance. In general, some form of light wave communication, such as infrared radiation, is utilized in order to localize the communication and minimize eavesdropping.
It is additionally desired that such communication operate in a full duplex mode permitting simultaneous transmission and reception at the portable unit as is typical of conventional telephone head sets. These system requirements combine to place certain constraints on such a portable communication system. Battery drain in the portable unit limits its useful life between replacement or recharges. Isolation of the light wave transmit and receive signal paths is necessary so as to avoid interference between the paths. Loss of sensitivity results from detuning of system parameters, and audio feedback due to coupling into the two wire telephone network. A portable hand held unit can suffer from blockage of the light wave signal by head and hair shadowing during use.
Battery drain is a significant problem because in order to ensure adequate signal strength over a wide range, the system must be capable of emitting a high level of modulated light. In most situations, the portable unit will be much closer to the network, not requiring such high emission energies and therefore wasting substantial battery power. The required level of modulated light at the photo receiver input (and therefore, the transmitter power requirement) depends on the maximum acceptable background noise as perceived by the user during conversation pulses.
Because both transmit and receive channels are simultaneously operative, and because the receive photo sensors must be sensitive to very low light levels, a substantial potential for interference results from their proximity to the high level emitted radiation from the portable, or network associated transmitting emitters. The photo sensors at the receiver must be able to discriminate between the locally emitted channel, at many orders of magnitude greater intensity, and the incoming signal to which they are intended to be solely responsive.
For reasons of efficiency narrow band pulse rate modulation is typically utilized for the optical transmit and receive links between the portable unit and the network associated receiving and transmitting equipment. Because of the narrow bandwidth thus utilized, and potential abuse to which the portable unit may be subjected in dropping or other accidents, slight detune of the equipment can produce an apparent loss in sensitivity and inadequacy of the portable unit in operating at distances.
Finally, in order to isolate signals being applied by the system to the telephone network from those being received from the telephone network for transmission to the portable unit, a hybrid coupling scheme must be utilized that effectively isolates these two signal flow paths. Such high isolation coupling systems are typically formulated of multiple passive balancing elements including inductors which are both costly and bulky and generally undesirable in modern light weight communication systems.